1. Field of the Invention
The present invention relates to a backing plate for a drum brake used for braking an automobile or the like.
2. Related Art
Drum brakes are used extensively as brakes for automobiles, and this brake is a device for braking a wheel by causing a lining of a brake shoe to be pressed against an inner peripheral surface of a brake drum coupled with the wheel and rotating together the same, thereby braking the brake drum by means of a frictional force occurring between the two members.
The conventional drum brake is arranged as illustrated in FIGS. 1 and 2. FIG. 1 is a front elevational view of a backing plate with components mounted thereon, and FIG. 2 is a side elevational view of an anchor as viewed in the direction of II in FIG. 1.
The conventional drum brake shown in FIG. 1 as disclosed, for example, in U.S. Pat. No. 5,038,898 is provided with a backing plate 1 made of a steel plate. A brake drum 2 is superposed on the backing plate 1 in a state in which an uprightly bent edge 1a of the backing plate 1 is inserted in a groove formed in a peripheral end face of the brake drum 2. The drum brake is also provided with an axle hole 3 for insertion of an axle therethrough and one of a plurality of bolt insertion holes 4 for securing the backing plate 1 to a chassis and a hole 5 for fitting and fixing a proximal portion of a wheel cylinder 6. A brake shoe 7 is secured loosely to the backing plate by means of a hold-down device 8, and the brake shoe 7 has one end 7a engaged with an anchor 9 and the other end 7b engaged with a piston rod 6a of the wheel cylinder 6. The anchor 9 is secured to the backing plate by means of pins 9a.
Reference numerals 10 and 11 denote return springs for the brake shoes 7, and numeral 12 denotes a parking brake lever having one end pivotally secured to one of the brake shoes 7 by means of a shaft 13, a cable (not shown) guided by a cable guide 14 being connected to the other end thereof. Numeral 15 denotes a hole for insertion of the cable, and numeral 16 denotes a dust shield welded to the backing plate for preventing the entry of dust and water into the brake drum.
Since the ends 7a of the brake shoes 7 tend to float away from the backing plate 1 during braking, an arrangement is provided such that, as shown in FIG. 2, a retainer plate 9c which is larger than an anchor piece 9b is superposed on the anchor piece 9b, and these members are connected by means of the pins 9a, so as to prevent the floating of the ends 7a.
In the drum brake thus constructed, if pressure oil is supplied to the wheel cylinder 6, and the piston rod 6a is thereby pushed out, the brake shoes rotate with the anchor 9 as their fulcrum. This, in turn, causes linings 7c to be pressed against the inner peripheral surface of the brake drum 2, thereby effecting braking. If the pressure oil is discharged from the wheel cylinder 6, the brake shoes 7, 7 return to their original positions by being pulled by return springs 10, 11, thereby releasing the linings 7c from the brake drum so as to stop the braking.
FIG. 3 is a front elevational view of the conventional backing plate which is shown by omitting the wheel cylinder, brake shoes, parking brake lever, and the like. FIG. 4 is a cross-sectional view taken along the line IV--IV of FIG. 3. FIG. 5 is a cross-sectional view taken along the line V--V of FIG. 3. The entry of dust into the brake drum is stopped as the rim 1a of the backing plate 1 is fitted in the groove provided in the end face of the brake drum. However, in order to render this mechanism more reliable, an annular dust shield 16 is welded onto the outer periphery of the illustrated backing plate 1.
Thus, various components such as the wheel cylinder, anchor, brake shoes, parking brake lever, cable guide, and dust shield are mounted on the backing plate 1 and, conventionally, the backing plate is formed through a multiplicity of processes by using a steel plate, and the respective components are mounted thereon.
Although the casting of a backing plate by means of a light metal or a light alloy is described in Unexamined Japanese Utility Model Application No. Sho. 55-97230, the technique disclosed therein is such that components-fixing bolts are threadably engaged with or secured to inner portions of the backing plate to prevent the bolts from being exposed to outside the backing plate, so that it is nonanalogous art of the present invention.
Various measures for making the vehicle body lightweight have been devised to improve the performance of automobiles, but if the backing plate is formed of a steel plate, the weight unavoidably becomes large.
That is, as the backing plate, portions fringed with slant lines in FIGS. 3 and 4 need to have their strength increased, but the remaining portions do not require large strength. In the case of the backing plate made of the steel plate, however, since the thickness is uniform, the thickness cannot be changed depending on portions, so that the backing plate is provided with unnecessary weight. In addition, an attempt to make the thickness of the overall backing plate small by using a high tension steel plate has been put to practical use, but there are limits in the reduction of weight according to that method.
At the same time, in addition to press operations involving a multiplicity of processes, since the components are fixed by means of jointing processes such as welding and caulking, the number of working processes increases, the above-described method is disadvantageous in the light of the production efficiency.